Abstract:

The invention provides a method of trimming a structure that includes a
first wafer bonded to a second wafer, with the first wafer having a
chamfered edge. The method includes a first trimming step carried out
over a first depth that includes at least the thickness of the first
wafer and over a first width determined from the edge of the first wafer.
A second trimming step is then carried out over a second depth that
includes at least the thickness of the first wafer and over a second
width that is less than the first width.

Claims:

1.-14. (canceled)

15. A method of trimming a structure while reducing applied mechanical
friction, with the structure comprising a first wafer bonded to a second
wafer, with the first wafer having a chamfered edge, which method
comprises: a first trimming step carried out over a first depth
comprising the thickness of the first wafer and also being carried out
over a first predetermined width from the edge of the first wafer; and at
least one second trimming step carried out over a second depth comprising
any remaining thickness of the first wafer, while being carried out over
a second width that is less than the first width; wherein the reduced
application of mechanical friction reduces heating and stresses such that
macro and micro peel-off phenomena of the bonded wafers is limited
compared to complete mechanical machining of the first wafer.

16. The method according to claim 15, wherein the second depth is less
than the first depth.

17. The method according to claim 15, wherein the portion of the
thickness of the second wafer withdrawn during the first trimming step is
in the range of 10 μm to 30 μm.

18. The method according to claim 15, wherein the portion of the
thickness of the second wafer withdrawn during the second trimming step
is in the range of 0 to 10 μm.

19. The method according to claim 15, wherein the first trimming step is
carried out over a first width in the range 2 mm to 10 mm for a wafer
having a diameter of between 100 and 300 mm.

20. The method according to claim 15, wherein the second trimming step is
carried out over a second width in the range of 0.1 mm to 2 mm.

21. The method according to claim 15, wherein the first wafer comprises
electrical components.

22. The method according to claim 15, wherein at least one of the
trimming steps is carried out with a grinder comprising grooves on its
lower surface.

23. A method of producing a three-dimensional composite structure which
comprises at least one step of producing a layer of electrical components
on one face of a first wafer, a step of bonding the face of the first
wafer comprising the layer of electrical components onto a second wafer,
and a step of trimming at least the first wafer in accordance with the
method of claim 15.

24. The method according to claim 23, wherein the step of thinning the
first wafer is conducted after bonding.

25. The method according to claim 23, which further comprises a step of
producing a second layer of electrical components on the face of the
first wafer opposite to the face comprising the first layer of electrical
components.

26. The method according to claim 23, which further comprises a step of
forming a layer of oxide on the face of the first wafer comprising the
first layer of electric components before bonding.

27. The method according to claim 23, wherein the first wafer is a SOI
type structure.

28. The method according to claim 23, wherein at least the first layer of
electrical components comprises image sensors.

29. In a method of trimming a layer of a bonded structure by mechanical
machining, wherein the structure comprises a first wafer bonded to a
second wafer, the improvement which comprises conducting the mechanical
machining of the first wafer over a predetermined depth and also being
carried out over a first predetermined width from the edge of the first
wafer; and replacing part of the mechanical machining with non-mechanical
trimming over at least any remaining thickness of the first wafer while
being carried out over a second width that is less than the first width;
wherein the trimming results in a reduced application of mechanical
friction to reduce heating and stresses such that macro and micro
peel-off phenomena of the bonded wafers is limited compared to complete
mechanical machining of the first wafer.

Description:

FIELD OF THE INVENTION AND PRIOR ART

[0001] The present invention relates to the field of producing multilayer
semiconductor structures or substrates (also termed multilayer
semiconductor wafers) produced by transfer of at least one layer onto a
support. The transferred layer is formed by molecular bonding of a first
wafer onto a second wafer or support, the first wafer generally being
thinned following bonding. The first wafer may also include all or part
of a component or a plurality of microcomponents, as happens with
three-dimensional (3D) integration of components, which requires transfer
of one or more layers of microcomponents onto a final support, and also
as happens with circuit transfer as, for example, in the fabrication of
back lit imaging devices.

[0002] The edges of the wafers used to form the transferred layers and the
supports generally have chamfers or edge roundings serving to facilitate
their manipulation and to avoid breakages at the edges that could occur
if those edges were to project, such breakages being sources of particles
that contaminate the wafer surfaces. The chamfers may be rounded and/or
bevelled in shape.

[0003] However, the presence of such chamfers prevents good contact
between the support and the wafer at their peripheries. As a result, a
peripheral zone exists on which the transferred layer is not bonded or
not properly bonded to the support substrate. This peripheral zone of the
transferred layer must be eliminated since it is liable to break in an
uncontrolled manner and contaminate the structure with unwanted fragments
or particles.

[0004] Thus, once the wafer has been bonded to the support and after any
necessary thinning thereof, the transferred layer is then trimmed in
order to remove the peripheral zone over which the chamfers extend.
Trimming is usually carried out essentially by mechanical machining, in
particular by abrasion or grinding from the exposed surface of the
transferred layer up to the support.

[0005] However, such trimming causes problems with peel-off, both at the
bonding interface between the transferred layer and the support and in
the transferred layer itself. More precisely, at the bonding interface,
peel-off problems correspond to delamination of the transferred layer
over certain zones in the vicinity of the periphery of the layer, which
delamination may be qualified as macro peel-off. The bonding energy is
lower near the periphery of the layer because of the presence of the
chamfers. As a consequence, grinding in this region may cause partial
detachment of the layer at its bonding interface with the support
substrate. Said detachment is more probable when the transferred layer
includes components. High temperature anneals, normally carried out after
bonding to reinforce the bonding interface, are not used when components
are present in the transferred layer since components cannot withstand
the temperatures of such anneals.

[0006] Further, when the layer comprises components such as circuits,
contacts, and in particular zones formed from metal, grinding may cause
delamination at the motifs of the components present in the transferred
layer, which delamination may be qualified as micro peel-off.

[0007] Such phenomena of macro and micro peel-off occur beyond a certain
level of heating and/or mechanical stress in the structure during the
trimming step. This level is frequently attained during complete trimming
of the transferred layer.

SUMMARY OF THE INVENTION

[0008] The aim of the invention is to overcome the disadvantages mentioned
above by proposing a method of trimming a structure comprising a first
wafer bonded to a second wafer, the first wafer having a chamfered edge,
the method comprising: [0009] a first trimming step carried out over a
first depth comprising the thickness of the first wafer, said first
trimming step also being carried out over a first predetermined width
from the edge of the first wafer; and [0010] at least one second trimming
step carried out over a second depth comprising at least the thickness of
the first wafer, said second trimming step also being carried out over a
second width that is less than the first width.

[0011] Thus, by carrying out a first trimming step as close as possible to
the edge of the first wafer and over a predetermined width, the first
wafer is attacked while remaining relatively far away from the components
thereof. This limits heating and/or stresses in the structure even when
trimming is intense, i.e. penetrating significantly into the second
wafer.

[0012] Further, the heating and/or stresses are also limited during the
second trimming step, even though said second trimming step is carried
out at a distance that is further from the edge of the first wafer, i.e.
close to the components. In fact, the material to be removed during the
second trimming step is reduced because of the portion that has already
been removed during the first trimming step.

[0013] As a result, the two trimming steps of the method of the invention
mean that complete trimming of at least the first wafer can be carried
out while substantially reducing the phenomena of macro and micro
peel-off that normally appear during single-step trimming.

[0014] In accordance with one aspect of the invention, the second trimming
step is carried out over a second depth that is less than or equal to the
first depth over which the first trimming step is carried out.

[0015] In accordance with another aspect of the invention, the portion of
the thickness of the second step withdrawn during the first trimming step
is in the range 10 μm [micrometer] to 30 μm.

[0016] In accordance with another aspect of the invention, the portion of
the thickness of the second wafer withdrawn during the second trimming
step is in the range 0 to 10 μm.

[0017] In accordance with yet another aspect of the invention, the first
trimming step is carried out over a first width in the range 2 mm
[millimeter] to 10 mm, preferably in the range 2 mm to 6 mm, while the
second trimming step is carried out over a second width in the range 0.1
mm to 2 mm.

[0018] The present invention also provides a method of producing a
three-dimensional composite structure comprising at least one step of
producing a layer of components on one face of a first wafer, a step of
bonding the face of the first wafer comprising the layer of components
onto a second wafer, and a step of trimming at least the first wafer
carried out in accordance with the trimming method of the invention.

[0019] The use of the trimming method of the invention means that
three-dimensional structures can be produced by stacking two or more
wafers, minimizing the risks of delamination both at the bonding
interfaces between the wafers and at the component layers. One of the
component layers may include image sensors.

[0021] FIGS. 2A to 2E are diagrammatic views of a trimming method in
accordance with one implementation of the invention;

[0022]FIG. 3 is a flow diagram of the steps carried out during the method
illustrated in FIGS. 2A to 2E;

[0023] FIGS. 4A to 4F are diagrammatic views showing the production of a
three-dimensional structure employing the trimming method of the present
invention;

[0024]FIG. 5 is a flow diagram of the steps carried out during production
of the three-dimensional structure illustrated in FIGS. 4A to 4F;

[0025]FIG. 6 is a view showing the lower surface of the grinder used in
FIGS. 4D and 4E.

DETAILED DESCRIPTION OF IMPLEMENTATIONS OF THE INVENTION

[0026] The present invention is of general application to trimming a
structure comprising at least two wafers assembled together by molecular
bonding or any other type of bonding such as anodic bonding, metallic
bonding, or bonding with adhesive, it being possible for components to be
formed beforehand in the first wafer that is then bonded to the second
wafer that constitutes a support. The wafers are generally of circular
outline, possibly with different diameters, in particular diameters of
100 millimeters (mm), 200 mm, or 300 mm. The term "components" as used
here means any type of element produced with materials that differ from
the material of the wafer and that are sensitive to the high temperatures
normally used to reinforce the bonding interface. These components
correspond in particular to elements forming all or a portion of an
electronic component or a plurality of electronic microcomponents, such
as circuits or contacts or active layers that may be damaged or even
destroyed if they are exposed to high temperatures. The components may
also correspond to elements, motifs, or layers that are produced with
materials with expansion coefficients different from that of the wafer
and that, at high temperature, are liable to create different degrees of
expansion in the wafer, which may deform and/or damage it.

[0027] In other words, when the first wafer include such components, it
cannot undergo high temperature anneals after bonding. As a consequence,
the bonding energy between the wafers is limited, which renders the
resulting structure rather more sensitive to the phenomenon of macro
peel-off during mechanical trimming, as described above. Further, as
explained above, the trimming may also cause micro peel-off,
corresponding to delamination in the first wafer at the components
(detachment in one or more of the stacks forming the components in the
first wafer).

[0028] More generally, the invention is of particular application to
assembled structures that cannot be subjected to a high temperature
bonding anneal, as also applies with heterostructures formed by an
assembly of wafers with different expansion coefficients (for example
silicon-on-sapphire, silicon-on-glass, etc). It may also apply to more
standard silicon-on-insulator (SOI) type structures, namely SOI
structures in which the two wafers are composed of silicon. For this type
of structure, the invention is of particular application to the formation
of structures that have a layer thickness of more than 10 micrometers
(μm), or that comprise a stack of layers of different natures. In
fact, it has been observed that these structures are liable to be damaged
during the trimming step when said trimming is carried out using the
known prior art technique.

[0029] To this end, the present invention proposes carrying out
progressive trimming from the edge of the first wafer. More precisely, as
explained below in more detail, the trimming method of the invention is
carried out in at least two steps, namely a first trimming step carried
out as close as possible to the edge of the wafer and a second trimming
step carried out at a distance further from the edge of the first wafer,
i.e. over a portion that is closer to the components of the wafer.

[0030]FIG. 1 is a top view showing a structure 15 comprising a wafer 10
bonded to a subjacent support (not shown). The wafer 10 includes
components 11 formed in a zone 14 termed the "useful zone" that covers
the major portion of the surface of the wafer with the exception of an
annular exclusion portion with a width l3 corresponding to the
distance between the edge 10a of the wafer 10 and the frontier of the
useful zone 14. The annular exclusion portion comprises at least the zone
over which the chamfers of the wafer extend. This annular portion may be
divided into first and second annular zones 12 and 13. The first annular
zone 12 with width l1 is the zone that is closest to the edge 10a of
the wafer. It is on this first zone 12 that the first trimming step of
the method of the invention is carried out. This first zone is relatively
distanced from the useful zone 14 including the components 11, and so
trimming may be carried out in the structure without running the risk of
macro or micro peel-offs. The second annular zone 13 with width l2
that is less than the width l1 is further from the edge 10a of the
wafer, i.e. closer to the useful zone 14. However, since a large quantity
of material has already been withdrawn during the first trimming step,
heating and stresses are limited during the second step of trimming of
the second annular zone 13. Thus, any macro and/or micro peel-off
phenomena that could occur during trimming are limited.

[0031] During the second trimming step, heating and stresses may be
further reduced by carrying out trimming over a depth that is less than
the depth over which the first trimming step is carried out.

[0032] In order to further limit heating and stresses during trimming, the
method of the invention may also be carried out in more than two steps,
for example three or four trimming steps. Under such circumstances, each
of the successive trimming steps is carried out over a width that is less
than or identical to that of the preceding trimming step. The trimming
depth for each step is preferably but not exclusively smaller than that
of the preceding trimming step.

[0033] One implementation of a trimming method is described below with
reference to FIGS. 2A to 2E and 3.

[0034] As can be seen in FIG. 2A, a structure 100 to be trimmed is formed
by assembling a first wafer 101 of the same type as that of FIG. 1 with a
second wafer 102, for example formed from silicon. The first and second
wafers 101 and 102 have the same diameter here. They could, however, have
different diameters. In the example described here, assembly is carried
out using the molecular bonding technique that is well known to the
skilled person. It should be recalled that the principle of molecular
bonding is based on bringing two surfaces into direct contact, i.e.
without using a specific bonding material (adhesive, wax, solder etc).
Such an operation requires that the surfaces to be bonded are
sufficiently smooth, free from particles or contamination, and that they
are brought sufficiently close together to allow contact to be initiated,
typically to a distance of less than a few nanometers. Under such
circumstances, forces of attraction between the two surfaces are high
enough to cause molecular bonding (bonding induced by the set of
attractive forces (van der Waals forces) due to electrons interacting
between atoms or molecules of the two surfaces to be bonded together).

[0035] Adhesion between the two wafers is carried out at a low temperature
so as not to damage the components and/or the first wafer. More
precisely, after bringing the wafers into contact at ambient temperature,
a bonding reinforcement anneal may be carried out, but at a temperature
of less than 450° C., beyond which temperature certain metals such
as aluminum or copper begin to creep.

[0036] A bonding layer 107 of the oxide layer type is formed on the
bonding face of the first wafer 101 and/or on the second wafer before
bringing it into contact with the second wafer 102. The first wafer 101
comprises a layer of components 103 and has a chamfered edge, i.e. an
edge comprising an upper chamfer 104 and a lower chamfer 105. In FIG. 2A,
the wafers have rounded chamfers. However, the wafers may also have
chamfers or edge roundings with different shapes such as in the form of a
bevel. In general, the term "chamfered edge" means any wafer edge at
which the ridges have been bevelled so that contact between the two
wafers close to their periphery is poor.

[0037] The wafers 101 and 102 are assembled one against the other by
molecular bonding to form the structure 100 (step S1, FIG. 2B). Depending
on the initial thickness of the first wafer 101, this may be thinned in
order to form a transferred layer 106 with a predetermined thickness
e1 (step S2, FIG. 2C), for example approximately 10 μm. The
thickness e1 is measured between the upper face and the lower face
of the layer or the wafer beyond the chamfered edge. This thinning step
is preferably carried out before the trimming operation. Thinning of the
first wafer, however, is still optional and trimming of the first wafer
may be carried out without carrying out a prior thinning step.

[0038] Next, trimming of the structure 100 is carried out, principally
consisting in eliminating an annular portion of the layer 106 comprising
the chamfer 105, the chamfer 104 having been eliminated during thinning
of the first wafer 101. In accordance with the invention, trimming
commences with a first trimming step carried out over a width ld1
from the edge of the first layer 106 that corresponds to the edge of the
first wafer 101 (step S3, FIG. 2D). For wafers with a diameter of 100 mm,
200 mm and 300 mm, the trimming width ld1 is generally in the range
2 mm to 10 mm, preferably in the range 2 mm to 6 mm. Trimming is carried
out by action or mechanical machining from the upper face of the layer
106 (edge grinding). The mechanical action may be exerted using a grinder
or any other tool that is capable of mechanically wearing away the
material of the layer.

[0039] During said first trimming step, the structure 100 is attacked over
a depth Pd1, defined from a reference plane corresponding to the
bonding interface (in this instance the plane of contact between the
bonding layer 107 and the bonding face of the second wafer 102). The
depth Pd1 comprises the thickness e1 of the layer 106, the
thickness e2 of the bonding layer 107 and a thickness e3
corresponding to a portion of the thickness of the second wafer 102. The
thickness e3 is in the range 10 μm to 30 μm. In FIG. 2D, the
flank of the trimmed layer 106 is shown in a diagrammatic manner as being
perpendicular to the plane of the substrate. However, depending on the
type of grinder used, the profile of the trimming flank may have
different shapes that are not entirely rectilinear, such as a slightly
inwardly curved shape. In particular, such inwardly curved flanks are
obtained when the grinder or trimming wheel is provided with grooves over
at least one of these faces. It appears that the presence of such grooves
encourages evacuation of the eliminated material and circulation of
liquid (generally water) dispensed over and close to the wheel during the
trimming operation. This further limits heating/stresses at the wafer
edge and can further improve the trimming quality. In circumstances where
the trimmed flank of the layer or wafer does not have a near rectilinear
profile, the widths of the trimming steps (such as widths ld1 and
ld2) correspond at least to the widths with which the wafer or layer
is attacked (the trimming width can then be slightly reduced during
trimming).

[0040] Trimming is then completed by a second trimming step that is also
carried out by mechanical action or machining (step S4, FIG. 2E). This
second trimming step is carried out from a predetermined distance from
the edge of the layer 106 corresponding to the trimming width ld1 of
the first step. For wafers with a diameter of 100 mm, 200 mm, and 300 mm,
the trimming width ld2 is generally in the range 0.1 mm to 0.2 mm.

[0041] In this second trimming step, the structure 100 is attacked over a
depth Pd2 comprising at least the thickness e1 of the layer
106. The depth Pd2 may also comprise a thickness e4
corresponding to a portion of the thickness of the second wafer 102. In
the example described here, the thickness e4 is less than the
thickness e3. It is in the range 0 to 10 μm, for example 5 μm.
As indicated above, the thickness e4 may also be greater than or
equal to the thickness e3.

[0042] A particular but not exclusive field for the trimming method of the
present invention is that of producing three-dimensional structures.

[0043] A method of producing a three-dimensional structure by transfer
onto a support of a layer of microcomponents formed on an initial
substrate in accordance with an implementation of the invention is
described below in relation to FIGS. 4A to 4G and 5.

[0044] Producing the three-dimensional structure starts with the formation
of a first series of microcomponents 204 on the surface of a first wafer
200 the edge of which has an upper chamfer 206 and a lower chamfer 205
(FIG. 4A, step S1). In the example described here, the first wafer 200 is
a multilayer SOI type structure, i.e. it comprises a layer of silicon 201
disposed on a substrate 203, also of silicon, a buried oxide layer 202
(for example a layer of SiO2) being present between the layer 201
and the substrate 203. The wafer 200 has a thickness in the range
approximately 600 μm to 900 μm. For a wafer 200 mm in diameter (8
inches), the standard thickness is 725 μm.

[0045] The microcomponents 204 are formed by photolithography using a mask
that can define zones for the formation of motifs corresponding to the
microcomponents to be produced.

[0046] The face of the first wafer 200 comprising the microcomponents 204
is then brought into intimate contact with a face of a second wafer 300
(step S2, FIG. 4B) with a view to bonding by molecular bonding. The wafer
300 has a thickness of approximately 725 μm. In the same manner as the
first wafer 200, the edge of the second wafer 300 has an upper chamfer
301 and a lower chamfer 302. A layer of oxide 207, for example formed
from SiO2, is also formed on the face of the first wafer 200
comprising the microcomponents 204. In the example described here, the
first and second wafers 200, 300 have a diameter of 200 mm.

[0047] After bonding, and as can be seen in FIG. 4C, the first wafer 200
is thinned to withdraw a portion thereof present above the layer of
microcomponents 204 (step S3), here the substrate 203. At this stage of
the method, the buried layer 202 is preferably retained in order to
protect the components from possible contamination, particles, etc. The
first wafer 200 may be thinned, in particular by a step of grinding or
chemical-mechanical polishing (CMP) of the substrate 203, stopping 50
μm from the bonding interface, followed by a step of chemical attack
up to the buried oxide layer 202, for example by etching with
tetramethylammonium hydroxide (TMAH). Thinning may also be carried out by
cleavage or fracture along a plane of weakness previously formed in the
wafer 200 by atomic implantation. Advantageously, the buried insulating
layer 202 is used to define the thickness of the remaining wafer 200.
After the thinning step, the wafer 200 has a thickness e of approximately
10 μm. In other circumstances, its thickness may lie in the range 1
μm to 15 μm.

[0048] Thus, a composite structure 500 is obtained, formed by the second
wafer 300 and the layer 201 corresponding to the remaining portion of the
first wafer 200.

[0049] In accordance with the invention, the first step of mechanical
trimming of the structure 500 is carried out, consisting of eliminating
an annular portion of the wafer 200 (step S4, FIG. 4D). This first
trimming step is carried out using a grinder 400, the structure 500 being
held in a rotating plate (not shown). As can be seen in FIG. 6, the
grinder 400 has a lower face that is structured due to the presence of
grooves 410. As indicated above, it has been observed that a grinder with
such a structured face can limit heating and stresses. Clearly, trimming
may also be carried out with grinders that do not have such structured
faces.

[0050] During this first trimming step, the structure 500 is attacked over
a width ld1 in the range 2 mm to 10 mm and over a depth Pd1
comprising the thickness e1 of the remaining portion of the first
wafer 200, the thickness e2 of the oxide layer 207 and a thickness
e3 corresponding to a portion of the thickness of the second wafer
300, said thickness e3 being in the range 10 μm to 30 μm.
Trimming is then completed by the second trimming step carried out from a
predetermined distance from the edge of the wafer 200 corresponding to
the trimming width ld1 of the first step and over a width ld2
in the range 0.1 mm to 2 mm (step S5, FIG. 4E). In this second trimming
step, the structure 500 is attacked over a depth Pd2 comprising the
thickness e1 of the remaining portion of the first wafer 200, the
thickness e2 of the oxide layer 207 and a thickness e4
corresponding to a portion of the thickness of the second wafer 300 in
the range 0 to 10 μm, for example 5 μm.

[0051] Once trimming of the structure 500 has been terminated, after
having withdrawn the layer 202, a second layer of microcomponents 214 is
formed at the exposed surface of the layer 201 (FIG. 4F, step S6). In the
example described here, the microcomponents 214 are formed in alignment
with the buried microcomponents 204. A photolithography mask is used for
this purpose; it is similar to that used to form the microcomponents 204.

[0052] In a variation, the three-dimensional structure is formed by a
stack of layers, i.e. by transfer of one or more additional layers onto
the layer 201, each additional layer being in alignment with the directly
adjacent layer or layers. Each additional layer is trimmed progressively
using the trimming method of the invention. Further, before each transfer
of an additional layer, it is possible to deposit a layer of oxide on the
exposed layer, for example a layer of tetraethyloxysilane (TEOS) oxide,
in order to facilitate assembly and protect the trimmed zones (for which
the material of the subjacent wafer is exposed) from subsequent chemical
attacks. Alternatively, a single trimming operation may be carried out
after the set of layers has been transferred. The thicknesses e1 and
e2 of the materials eliminated during the trimming step of the
invention then correspond to the thicknesses included between the upper
surface of the upper layer of the stack and the stop interface for the
trimming step at or within the support wafer.

[0053] In accordance with a particular implementation, one of the layers
of microcomponents may in particular comprise image sensors.

[0054] In accordance with another implementation, the components have
already been formed in the second support wafer before assembly thereof
with the first wafer constituting the transferred layer.

[0055] In accordance with yet another implementation, the trimming steps
may include a first rough removal step, for example using a grinder as
represented in FIG. 4D, followed by a finer removal step, for example
using a wafer edge polish tool. This means that, after trimming, a wafer
edge with reduced roughness can be produced that is less susceptible of
having residual particles.